Known types of lens structures used in flash units for (digital or film) cameras are frequently “circular” lenses of normal or Fresnel type with circular or rectangular outlines. Although the brightness provided by such a flash unit may be relatively high, the uniformity of illumination across the field of view of the camera may be relatively low. The effect of such poor uniformity of illumination is especially pronounced in wide angle photographs. This may result in images of objects to the sides of the field of view and in the corners thereof being relatively dim or even invisible in the photograph.
Part of the reason for this is that camera fields of view are generally rectangular in shape whereas optics tend to be circular. Another part of the reason results from the emission profile of the flash unit or other light source. In the case of an isotropic light emitter without any optical system, the light flux at the corner a rectangular plane a fixed distance from the source is given by cos3θ that of the flux at the centre of the rectangular plane, where the angle θ measured at the light emitter is the angle between the normal to the centre of the rectangular plane and a line from the light emitter to the corner of the rectangular plane. In a typical example where the angle θ is approximately 37°, the resulting uniformity (ratio of flux in the corner to flux at the centre) is approximately 51%. In the case of a Lambertian light emitter such as a light emitting diode (LED) emitter, the uniformity is equal to cos4θ and, in the specific example of θ=37°, is approximately 42%.
In general, as is known, it is possible with narrower fields of view to obtain good uniformity of illumination using only converging optical elements. However, for wider fields, this is more difficult when the illumination from the light source on its own falls off at high angles, i.e. source only illumination becomes significantly less than that at the centre. For older types of flash, using flashbulbs, the source is most conveniently modelled as isotropic and illumination does not fall off very fast with angle onto a plane. However, for surface sources like LEDs, which are most conveniently modelled as Lambertian sources, illumination falls off much faster with angle and this makes it harder to obtain good uniformity at high angles. LEDs have better power characteristics than flash bulbs and thus are more likely to be used, for example, in mobile devices. Also, cameras in mobile devices are unlikely to have mechanical zooms or detachable lenses so that the fixed field of such a mobile camera is usually set to give a general-use field angle which may typically be around 70-75° or greater diagonal field. In addition, it is useful for the illumination plane to be somewhat larger than the actual camera field, to allow for design uncertainty and the fact the flash and camera are not in the same place on the device (parallax).
Lenses are generally provided in camera flash units in order to improve the brightness of illumination. Such arrangements improve the brightness in the centre of the field of view. However, the focussing nature of such lenses tends to direct light away from the edges of the field of view towards the centre thereof, increasing the central brightness but inevitably reducing uniformity of illumination across the field of view, especially for wide fields with Lambertian illuminators.
In this patent, a curved lens surface shall be referred to as “spherical” if the Cartesian (X, Y, Z) shape of the curve of the surface satisfies the equation (Z is known as the sag of the surface):Z=R−√{square root over (R2−X2−Y2)}where R is the radius of curvature of the surface.
An “aspheric” surface is defined by:
  Z  =                              (                                    X              2                        +                          Y              2                                )                /        R                    1        +                              1            -                                          (                                  1                  +                  K                                )                            ⁢                                                (                                                            X                      2                                        +                                          Y                      2                                                        )                                /                                  R                  2                                                                          +                  ∑        p            ⁢                          ⁢                                    A            p                    ⁡                      (                                          X                2                            +                              Y                2                                      )                          p            
where R is the radius of curvature at the centre, K is the conic constant and Ap are higher order coefficients for integer values of p>1.
An “asymmetric” surface is defined by the above equation where (X2+Y2) is replaced by (nX2+mY2) where n≠0, m≠0 and n≠m.
A “cylindrical” surface is where either n or m is zero.
An “anamorphic” surface is where the function describing the surface is other than above and may be the sum of two different aspheric, cylindrical or asymmetric functions or a mixture of these.
A “plane” surface or “optically flat surface” is a spherical surface where R is infinite.
Unless a type is specifically mentioned, a reference to a lens surface in the description may refer to any of the above types of surface.
U.S. Pat. No. 5,615,394 discloses the use of anamorphic lenses for camera flash assemblies. This arrangement has at least one surface of a lens assembly whose shape is modified away from a standard lens shape so as to control illumination. Such an arrangement uses all convergent elements and has problems with wide fields or Lambertian LED sources.
U.S. Pat. No. 5,160,192 discloses the use of an asymmetric ellipsoidal reflector (two identical half-ellipsodial mirrors arranged so that their axes are in different places with a flat mirror making up the gap) behind a flash bulb and a Fresnel lens condenser in order to improve the uniformity of illumination of a flash unit for a camera. This arrangement has all convergent elements and so has problems with wide fields or Lambertian LED sources. However, rear-mounted mirrors are of no use to surface emitters such as LEDs. Also, such an arrangement is of large size and is difficult to mount in a mobile device
U.S. Pat. No. 4,462,063 discloses the use of a spherical and aspheric lens assembly around a flash bulb in order to improve uniformity of illumination. Again, all convergent elements are used and this has problems with wide fields or Lambertian LED sources. This arrangement is specific to lamp bulbs, is of large size and is difficult to mount in a mobile device
U.S. Pat. No. 6,088,540 discloses an arrangement including an additional element utilising two surfaces, one a normal converging surface and one a Fresnel-like surface utilising total internal reflection (TIR) in order to improve focussing power. Such an arrangement provides high brightness but poor uniformity of illumination, with the production of one or more “bright spots” being visible. This arrangement is specific to flashlamps and requires a linear volume source and back reflector. The TIR element is designed to bring high angle light more forward and in this respect it is a convergent element.
U.S. Pat. No. 5,778,264 discloses a mirror system for widening an illumination area. However, such an arrangement is relatively bulky and is inconvenient or unsuitable for relatively compact portable or mobile devices. Such an arrangement is in the form of a large area adapter, is bulky and is difficult to apply to a small mobile device. Further, it does not improve brightness of illumination.
US 20020009297, US 20010028792, U.S. Pat. No. 6,496,650, U.S. Pat. No. 6,771,898 and EP 0756195 disclose various techniques for changing the focus of a flash beam. These mainly concentrate on changing the illumination area in response to a camera zoom. US 20020009297 describes a switching lens system for altering the divergence of the light. US20010028792, U.S. Pat. No. 6,496,650 & U.S. Pat. No. 6,771,898 also describe a shifting convergent lens system. EP0756195 describes a system whereby the flash fires twice to ascertain the best illumination parameters and these are sent to the flash unit. All of these arrangements concern modifications to existing all-convergent optical system designs with the inherent issues mentioned above. Adaptable flash zooms are more complex technically and difficult to apply to small mobile devices.
GB 1 391 677 discloses an optical device comprising a converging reflector and a diverging element formed by a four-sectored prism arrangement.
GB 818,229 discloses a cinema projector having a light source with a condensing system. A cylindrically diverging lens followed by a cylindrically converging lens with the axes parallel is provided within the condensing system.
U.S. Pat. No. 5,769,521 discloses an arrangement for homogenising laser radiation. The arrangement comprises a “lenticular” in the form of “acentric” lens segments which are optically converging. This is followed by a converging “collecting” lens.
U.S. Pat. No. 5,553,174 discloses an arrangement for circularising a light beam from a laser diode. The arrangement comprises a collimating lens followed by a cylindrically diverging minilens.
WO 97/38352 discloses a projector light source. A diverging lens is disposed ahead of a converging condensing lens. The diverging lens has a concave surface with peripheral regions of lower curvature than the central region.
GB 1,144,182 discloses an arrangement for homogenising a laser beam while maintaining its collimation. The device comprises two aspheric lenses. The first is largely diverging from the centre of the laser beam so as to provide more uniform illumination on the second lens. The second lens then re-collimates the light. All optical paths through the device have the same optical length so as to preserve the wavefront shape of the incident light.
U.S. Pat. No. 6,283,613 discloses an LED overhead traffic signal. In an array of LEDs, each LED is provided with a conical reflector and what is referred to as a Fresnel “lens”, although this latter device appears to be a Fresnel prism. The purpose of this device is effectively to deviate light downwardly.
JP 2003-331612 discloses an LED vehicle light. Each LED is disposed at one end of a cylindrical reflector. The outputs of the reflectors pass through converging Fresnel lenses and converging lenses.
GB 2 2258 659 discloses a light source for a barcode reader. The light source comprises one or more LEDs with the or each LED being disposed behind a one dimensional converging lens which differs from having a cylindrical curvature in that the curvature varies with distances from the longitudinal axis. This is followed by a semi-cylindrical lens having a radius of curvature which varies along its longitudinal axis. Thus, both lenses are convergent.
U.S. Pat. No. 4,737,896 discloses a backlight for an LCD TV. The backlight comprises a light source at the focus of a converging lens, which thus outputs a parallel light beam. This light beam is reflected perpendicularly by a microprism array to a “scattering” surface for supplying diffuse light to the LCD.
U.S. Pat. No. 4,510,560 discloses a surface light source intended to provide uniform illumination over a working surface. FIG. 16 of this document illustrates a flat output element having an internal surface of microprism structure. Below the centre of this is a light source with a concave converging mirror to reflect light away from the output surface. Below this structure is a curved reflecting structure for providing uniform illumination of the microprism surface, which then redirects output light. Directly below the light source is a hole in the reflecting structure. The edges of the hole are convexly reflective and hence diverging so as to reflect light to the centre of the output surface to fill the shadow caused by the small concave mirror.
US 2004/0131157 discloses an LED light source for use in setting up an X-ray machine. The LEDs have integral converging lenses and some embodiments have parabolic or ellipsoidal reflectors which are also converging. FIG. 17 of this document shows what appears to be a concavo-convex lens and FIG. 18 of this document shows a diverging lens.